1. Field of the Invention
The invention relates generally to rapidly solidified iron-rich metal alloys obtained by adding small amounts of boron to alloys having compositions similar to those of commercial precipitation-hardenable stainless steels. This invention also relates to the preparation of these materials in the form of powder and the consolidation of these powders (or, alternatively, the ribbon-like material obtained from melt-spinning) into bulk parts which are heat treated to have these desirable properties.
2. Description of the Prior Art
Rapid solidification processing techniques offer outstanding prospects for the creation of new breeds of cost-effective engineering materials with superior properties. (See Proceedings, Int. Conf. on Rapid Solidification Processing, Reston, Va., Nov. 1977, published by Claitor's Publishing Division, Baton Rouge, La., 1978.) Metallic glasses, microcrystalline alloys, highly supersaturated solid solutions and ultrafine grained alloys with highly refined microstructures, in each case often having complete chemical homogeneity, are some of the products that can be made utilizing rapid solidification processing (RSP). (See Rapidly Quenched Metals, 3rd Int. Conf., Vol. 1 & 2, B. Cantor, Ed., The Metals Society, London, 1978.)
Several techniques are well established in the state of the art to economically fabricate rapidly solidified alloys (at cooling rates of .about.10.sup.5 to 10.sup.7 .degree. C./sec) as ribbons, filaments, wire, flakes or powders in large quantities. One well known example is melt spin chill casting, whereby the melt is spread as a thin layer on a conductive metallic substrate moving at high speed (see Proc. Int. Conf. on Rapid Solidification Processing, Reston, Va., Nov. 1977, p. 246.)
The current technological interest in materials produced by rapid solidification processing, especially when followed by consolidation into bulk parts, may be traced, in part, to the problems associated with the chemical segregation that occurs in complex, highly alloyed materials during the conventional procedure of ingot casting and processing. During the slow cooling characteristic of casting processes, solute partitioning, i.e., macro- and micro-segregation within the different alloys phases present in these alloys, and the formation of undesirable, massive grain boundary eutectic phases can occur. Metal powders produced directly from the melt by conventional techniques, i.e., inert gas or water atomization of the melt, are usually cooled at rates three to four orders of magnitude lower than those that can be obtained by rapid solidification processing. Rapid solidification processing removes macro-segregation altogether and significantly reduces the spacing over which micro-segregation occurs, if it occurs at all.
The design of alloys made by conventional slow cooling processes is largely influenced by the corresponding equilibrium phase diagrams, which indicate the existence and coexistence of the phases present in thermodynamic equilibrium. Alloys prepared by such processes are in, or at least near, equilibrium. The advent of rapid quenching from the melt has enabled materials scientists to stray further from the state of equilibrium and has greatly widened the range of new alloys with unique structures and properties available for technological applications. Thus, it is known that the metalloid boron has only very low solid solubility in the transition metal Fe. Alloys of Fe containing significant amounts of boron, e.g., in the range of 1-2 wt%, prepared by conventional technology have, at most, limited usefulness because they are extremely brittle. This brittleness is due to a network of a hard and brittle eutectic boride phase present along the boundaries of the primary grains of the alloys.
The presence of these hard borides in these alloys could be advantageous if they could be made to be finely dispersed in the matrix metals in the same manner in which certain precipitates are dispersed in precipitation-hardened or dispersion-hardened commercial alloys based on Al, Cu, Fe, Ni, Co and the like.
Several classes of iron-rich alloys combining relatively high strength with corrosion resistance, collectively labelled the precipitation-hardenable (PH) stainless steels (see Handbook of Stainless Steels, D. Peckner and I. M. Bernstein, Eds., McGraw Hill Book Co., New York, 1977, p. 7-1), are commercially available. These are labelled the austenitic, martensitic, and semiaustenitic classes, of which the latter two are the most widely used.
The semiaustenitic precipitation-hardenable stainless steels, in their solution treated or annealed condition, are essentially austenitic but also contain 5 to 20% delta ferrite. They can be transformed to martensite through a series of thermal or thermomechanical treatments, and they can be further hardened, to their final strength level, by an aging treatment to precipitate intermetallic compounds.
The martensitic precipitation-hardenable stainless steels sustain the greatest volume of usage. After solution treatment, they are always in the martensitic condition at room temperature. Age hardening, carried out between 800.degree.-1250.degree. F., causes the precipitation of various intermetallic compounds. Typically, such martensitic PH stainless steels contain .about.0.03-0.13 wt% C, .about.12-17 wt% Cr, .about.4-9 wt% Ni and/or Co as well as lesser amounts of elements such as Mo, W, Al, Cu, Ti, Cb, V, Ta and N, at least some of which are generally included to produce the intermetallic precipitates.